There are plans to keep sha1s around in a database, but as far as I know, no way to transmit those, so they seem specific to individual forges. They can be recomputed, sure, but again, any signatures break and it's possible that in the case of an actual replacement, the recomputation is now wrong and not easily comparable. So what is the point?
This is not about forges, it is about the repo I have in a folder on my computer. The sha1 hashes shouldn't go away. Yes the forges also need to support this.